As a speaker system suitable applicable to a home theater, AV (audio and visual) system, etc., speaker arrays are disclosed in the Japanese Patent Application Laid Open Nos. 233591 of 1997 and 30381 of 1993. FIG. 1 shows one of the conventional speaker arrays, as a typical example. The speaker array generally indicated with a reference numeral 10 includes a plurality of speakers (speaker units) SP0 to SPn disposed in an array. In this speaker array, n=255 and each of the speakers has a diameter of several centimeters, for example. Thus, the speakers SP0 to SPn are actually disposed two-dimensionally in a plane. In the following description, however, it is assumed that the speakers SP0 to SPn are disposed in a horizontal line for the simplicity of illustration and explanation.
An audio signal is supplied from a source SC to delay circuits DL0 to DLn where it will be delayed by predetermined times τ0 to τn, respectively, the delayed audio signals are supplied to speakers SP0 to SPn, respectively, via power amplifiers PA0 to PAn, respectively. It should be noted that the delay times τ0 to τn given to the audio signal in the delay circuits DL0 to DLn will be described in detail later.
Thus, the sound waves delivered from the speakers SP0 to SPn will be combined together to provide a sound pressure to the listener wherever he or she positions himself or herself in relation to the speakers. On this account, in a sound field formed by the speakers SP0 to SPn as shown in FIG. 1, a predetermined sound pressure increasing point Ptg and predetermined sound pressure decreasing point Pnc are defined as follows:                Ptg: Point where the listener should be given as much sound as possible or the sound pressure should be increased more than in the surrounding        Pnc: Point where the listener should be given as less sound as possible or the sound pressure should be decreased more than in the surrounding.        
Generally, an arbitrary point can be taken as the sound pressure increasing point Ptg in a system shown in FIG. 2 or 3.
More specifically, on the assumption that in the system shown in FIG. 2, distances from the speakers SP0 to SPn to the sound pressure increasing point Ptg are L0 to Ln, respectively, and the acoustic velocity is s, the delay times τ0 to τn given to the sound waves in the delay circuits DL0 to DLn are set as follows in the system shown in FIG. 2:τ0=(Ln−L0)/s τ1=(Ln−L1)/s τ2=(Ln−L2)/s . . .τn=(Ln−Ln)/s=0
Thus, the audio signal from a source SC will be converted by the speakers SP0 to SPn into sound waves and the sound waves will be delivered from the respective speakers SP0 to SPn with delay times τ0 to τn, respectively. Therefore, all the sound waves will simultaneously arrive at the sound pressure increasing point Ptg and the sound pressure at the sound pressure increasing point Ptg will be higher than in the surrounding.
More specifically, in the system shown in FIG. 2, the distances from the speakers SP0 to SPn to the sound pressure increasing point Ptg are different from each other, which will cause a time lag from one sound wave to another. The time lag is compensated by a corresponding one of the delay circuits DL0 to DLn to focus the sound at the sound pressure increasing point Ptg. It should be noted that the system of this type will be referred to as “focusing type system” hereinafter and the sound pressure increasing point Ptg also be referred to as “focus” wherever appropriate hereinafter.
In the system shown in FIG. 3, the delay times τ0 to τn to be given to the sound waves in the delay circuits DL0 to DLn are so set that the phase wavefronts of the traveling waves (sound waves) from the speakers SP0 to SPn will be the same, to thereby make the sound waves directive and take direction toward the sound pressure increasing point Ptg as an intended direction. This system is also considered as a version of the focusing type in which distances L0 to Ln are infinitely large. It should be noted that the system of this type will be referred to as “directive type system” hereinafter and the direction in which the phase wavefronts of the sound waves are in a line be referred to as “intended direction” hereinafter.
In the speaker array 10, appropriate setting of the delay times τ0 to τn permits to form a focus Ptg at an arbitrary point within an a sound field and direct the sound waves in the same direction. Also, in both the above focusing and directive type systems, since outputs from the speakers SP0 to SPn are combined out of phase in any other position than the point Ptg, they will eventually be averaged and the sound pressure be lower. Further, in these systems, the sound outputs from the speaker array 10, once reflected by a wall surface, may be focused at the point Ptg and directed toward the point Ptg.
However, the aforementioned speaker array 10 is destined primarily to implement a sound pressure increasing point Ptg by focusing or directing the sound waves with the delay times τ0 to τn. The amplitude of an audio signal supplied to the speakers SP0 to SPn will only change the sound pressure.
On this account, the directivity of the speaker array may be utilized to lower the sound pressure at the sound pressure increasing point Ptg. For this purpose, the speaker array 10 may be rearranged for a main lobe to be formed in the direction of the sound pressure increasing point Ptg while reducing the side lobe or for null sound to be detected in the direction toward the sound pressure decreasing point Pnc, for example.
To this end, it is necessary to make the size of the entire speaker array sufficiently large in comparison with the wavelength of the sound wave by increasing the number n of the speakers SP0 to SPn. However, this is practically very difficult to implement. Otherwise, a change of sound pressure will have an influence on the sound pressure increasing point Ptg to which the sound waves are focused and directed.
Moreover, multi-channel stereo sound has to be taken in consideration for a home theater, AV system and the like. Namely, as the DVD players are more and more popular, multi-channel stereo sound sources are increasing. Thus, the user should provide as many speakers as the channels. However, a rather large space will be required for installation of so many speakers.
Also, to have the delay circuits DL0 to DLn delay an audio signal supplied from the source SC without degradation, each of the delay circuits DL0 to DLn have to be formed from a digital circuit. More particularly, the delay circuit may be formed from a digital filter. Actually, in many AV systems, since the source SC is a digital device such as a DVD player and the audio signal is a digital one, each of the delay circuits DL0 to DLn will be formed from a digital circuit in so many cases.
However, if each of the delay circuits DL0 to DLn is formed from a digital circuit, the time resolution of an audio signal supplied to the speakers SP0 to SPn will be limited by the digital audio signal and sampling period in the delay circuits DL0 to DLn and hence cannot be made smaller than the sampling period. It should be noted that when the sampling frequency is 48 kHz, the sampling period will be about 20.8 μsec and the sound wave will travel about 7 mm for one sampling period. Also, a 10-hz audio signal will be delayed by one sampling period equivalent to a phase delay of 70 deg.
Therefore, the phase of the sound wave from each of the speakers SP0 to SPn cannot sufficiently be focused at the point Ptg with the result that the size of the focus Ptg, that is, a sound image as viewed from the listener, will be larger or become not definite as the case may be.
Also, the sound wave phase will be less uneven in any place other than the focus Ptg and thus no sufficient reduction of the sound pressure can be expected in the other place than the point Ptg. Thus, the sound image will become large and not definite and will be less effective than usual.